Concrete slabs have varying thermal efficiencies

13 August 2018

New homes are generally built on concrete slabs, of which there are two main types. The first is the raft slab, which is a reinforced concrete slab with footings around the perimeter and typically some internal beams. The footings are cut into the soil and the slab sits with direct contact on the bearing soil for support and strength.

The second type, becoming more common, is the waffle pod slab, which has polystyrene “pods” beneath the slab, held in place by strip footings between the pods and an edge beam around the perimeter. A waffle pod slab has better thermal properties than a raft slab and is suited to colder climates. This is due to the bulk of the concrete floor being supported by the polystyrene pods and not having direct contact with the earth, apart from the inter-connecting strip footings.

Waffle pod slabs are available throughout Australia, although not all slab contractors will have experience in building them, according to Paal documentation manager Peter Aloisio. Factors that will also dictate whether certain areas are better for waffle pod slabs or raft slabs can be soil type or cyclonic areas or sloping sites.

Owner Preferences

Owner builders often have a preference for a type of slab, he says. They may favour a raft slab, but if their home plan then fails the required energy assessment, an upgrade to a waffle pod slab may be the solution.

Alternatively, the owner builder may achieve the required energy efficiency score by choosing more efficient glazing or other techniques. Your Paal housing consultant can suggest options and show their likely costs.

Costs of upgrades

An upgrade to a waffle pod slab may cost an additional $1000, while choosing the double glazing option could add $3000 to the cost of the home.

“A slab upgrade isn’t the only option when trying to improve a home’s energy efficiency,” Peter says. “As well as double glazing, there’s also the alternative of Low-E glass, or else bulk insulation along with “insulbreak” can be added to the external walls, if the house cladding is directly connected to the frame.

“Where an air gap is employed, as with brick veneer or aerated concrete panels, additional insulation won’t be needed, as air is a good insulator.”

Off the ground

Building on sloping blocks using piles, bearers and joists makes it harder to achieve thermal efficiency, as cold or warm air can circulate beneath the suspended flooring. The builder may need to install floor insulation such a fibreglass or insulated panels between floor joists, or improve the thermal properties of other elements of the home, such as the windows.

“If the slope is moderate, Paal usually recommends excavating to allow a level concrete slab. If the owner builder wants his slab to be elevated to give the home stature, this can be achieved by preparing the site so as to have areas of fill, instead of excavating into the site and lowering the home on the block, Peter says.